By Paul Rincon Science editor, BBC News

Nasa’s Kepler telescope has discovered more than 100 Earth-sized planets orbiting alien stars.

It has also detected nine small planets within the so-called habitable zone, where conditions are favourable for liquid water – and potentially life.

The finds are contained within a catalogue of 1,284 new planets detected by Kepler – which more than doubles the previous tally.

Nasa said it was the biggest single announcement of new exoplanets.

Space agency scientists discussed the new findings in a teleconference on Tuesday.

Statistical analyses of the expanding sample of worlds helps astronomers understand how common planets like our own might be.

Dr Natalie Batalha, Kepler mission scientist at Nasa’s Ames Research Center in California, said the calculations so far suggested there could be tens of billions of potentially habitable planets in the Milky Way.

“If you ask yourself where is the next habitable planet likely to be, it’s within about 11 light-years, which is very close,” said Dr Batalha.

Astronomers said they consider planets that are 1.6 times Earth’s radius or smaller are likely to be rocky, and may therefore be potential targets in the hunt for life.

Future telescopes such as the James Webb Space Telescope could analyse the light from the atmospheres of exoplanets for potential markers of biology.

Lead author Dr Timothy Morton, from Princeton University in New Jersey, said the overwhelming majority of exoplanets found by Kepler fell into the super-Earth (1.2-1.9 times bigger than the radius of Earth) and sub-Neptune sized (1.9-3.1 times bigger than Earth’s radius).

He noted that planets in this size range had no known analogues in our Solar System.

Scientists used a new statistical technique to validate the 1,284 new exoplanets. They folded in different types of information about the candidates from simulations, giving the astronomers a reliability score for each potential new world.

Candidates with a reliability greater than 99% were designated as “validated planets”.

Kepler employed the transit method to detect planets orbiting other stars. This involves measuring the slight dimming of a star’s light when an orbiting planet passes between it and the Earth.

The same orbital phenomenon was involved when Mercury passed across the face of the Sun on Monday 9 May.

The Kepler telescope, named after the Renaissance astronomer Johannes Kepler, was launched on 7 March 2009.

IMAGES: All Images copyright NASA

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